After attempting to update to MediaWiki 1.38, I also updated the Wikibase extension. Nevertheless, both the new and the old Wikibase are currently producing the same error, hinting at a problem with how Wikibase interacts with the new MediaWiki version 1.38.
Full error log:
[5fd0ce49dfdffcca08f1df96] /wiki/ProleWiki:Main_page Error: Class 'DataValues\Deserializers\DataValueDeserializer' not found
Backtrace:
from /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(264)
#0 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}()
#1 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService()
#2 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService()
#3 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService()
#4 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(361): Wikimedia\Services\ServiceContainer->get()
#5 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(860): Wikibase\Client\WikibaseClient::getDataValueDeserializer()
#6 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}()
#7 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService()
#8 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService()
#9 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService()
#10 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(155): Wikimedia\Services\ServiceContainer->get()
#11 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(1057): Wikibase\Client\WikibaseClient::getSingleEntitySourceServicesFactory()
#12 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}()
#13 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService()
#14 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService()
#15 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService()
#16 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(160): Wikimedia\Services\ServiceContainer->get()
#17 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(956): Wikibase\Client\WikibaseClient::getWikibaseServices()
#18 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}()
#19 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService()
#20 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService()
#21 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService()
#22 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(222): Wikimedia\Services\ServiceContainer->get()
#23 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(401): Wikibase\Client\WikibaseClient::getStore()
#24 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}()
#25 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService()
#26 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService()
#27 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService()
#28 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(170): Wikimedia\Services\ServiceContainer->get()
#29 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(826): Wikibase\Client\WikibaseClient::getEntityLookup()
#30 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}()
#31 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService()
#32 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService()
#33 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService()
#34 /var/www/prole/vendor/wikimedia/object-factory/src/ObjectFactory.php(211): Wikimedia\Services\ServiceContainer->get()
#35 /var/www/prole/vendor/wikimedia/object-factory/src/ObjectFactory.php(152): Wikimedia\ObjectFactory\ObjectFactory::getObjectFromSpec()
#36 /var/www/prole/includes/HookContainer/HookContainer.php(473): Wikimedia\ObjectFactory\ObjectFactory->createObject()
#37 /var/www/prole/includes/HookContainer/HookContainer.php(156): MediaWiki\HookContainer\HookContainer->getHandlers()
#38 /var/www/prole/includes/HookContainer/HookRunner.php(2858): MediaWiki\HookContainer\HookContainer->run()
#39 /var/www/prole/includes/parser/Parser.php(635): MediaWiki\HookContainer\HookRunner->onParserClearState()
#40 /var/www/prole/includes/parser/Parser.php(4833): Parser->clearState()
#41 /var/www/prole/includes/parser/Parser.php(926): Parser->startParse()
#42 /var/www/prole/includes/parser/Parser.php(4860): Parser->preprocess()
#43 /var/www/prole/includes/cache/MessageCache.php(1257): Parser->transformMsg()
#44 /var/www/prole/includes/language/Message.php(1474): MessageCache->transform()
#45 /var/www/prole/includes/language/Message.php(1007): Message->transformText()
#46 /var/www/prole/includes/language/Message.php(1071): Message->format()
#47 /var/www/prole/includes/OutputPage.php(1041): Message->text()
#48 /var/www/prole/includes/OutputPage.php(1090): OutputPage->setHTMLTitle()
#49 /var/www/prole/includes/page/Article.php(464): OutputPage->setPageTitle()
#50 /var/www/prole/includes/actions/ViewAction.php(80): Article->view()
#51 /var/www/prole/includes/MediaWiki.php(543): ViewAction->show()
#52 /var/www/prole/includes/MediaWiki.php(321): MediaWiki->performAction()
#53 /var/www/prole/includes/MediaWiki.php(912): MediaWiki->performRequest()
#54 /var/www/prole/includes/MediaWiki.php(563): MediaWiki->main()
#55 /var/www/prole/index.php(53): MediaWiki->run()
#56 /var/www/prole/index.php(46): wfIndexMain()
#57 {main}